1. Field of the Invention
The present invention relates to a dust-proof measure for a casing of an optical scanning apparatus used for an image forming apparatus such as a copying machine, a printer, a facsimile, or their multifunction peripheral.
2. Description of the Related Art
As an optical scanning apparatus used for an electrophotographic image forming apparatus, an optical scanning apparatus having the following configuration has been well-known. The optical scanning apparatus forms a latent image on a photosensitive member by deflecting a light beam emitted from a light source using a rotating polygonal mirror and guiding the deflected light beam onto the photosensitive member using an optical component including a lens or a mirror. FIG. 7 illustrates an outline of components of a conventional optical scanning apparatus that has generally been used. When dirt such as dust adheres to an optical component installed in the optical scanning apparatus, the dust adhering thereto blocks a light beam. Thus, an amount of the light beam on a photosensitive member decreases. With the decrease in the light amount of the light beam, an image is not output at a desired density. In recent years, amounts of fine dust and a chemical substance having a size of 1 μm or less in the air has increased. Deterioration in image quality due to dirt on the optical component has been becoming an issue more than before. Usually, the optical component serving as the component of the optical scanning apparatus is accommodated in a casing illustrated in FIGS. 8A, 8B, and 8C described below to protect the optical component from a substance causing dirt.
FIGS. 8A, 8B, and 8C are schematic configuration diagrams illustrating a retaining configuration of an optical component in a casing 185 of a conventional optical scanning apparatus. As a material for the casing 185, resin or a metal member is generally used, and is formed by a die. On the outer wall of the casing 185, a number of die cut holes H for forming an engaging portion 86b are formed. A plate spring for fixing an optical member is attached to the engaging portion 86b. FIGS. 9A and 9B illustrate how the die cut hole H is formed during the formation of the casing 185. The detailed description thereof will be given in an exemplary embodiment described below. In a conventional product, the die cut holes H are covered by affixing a number of sheet-type sealing members to the outer wall of the casing 185 to prevent the optical component from being soiled by dust, which has passed through the die cut holes H, adhering thereto. However, the casing 185 may have a number of ribs installed therein from the necessity of ensuring rigidity and may have a complicated shape due to an arrangement constraint of units around the image forming apparatus. If the ribs are formed at positions where they prohibit the sealing members from being affixed, the die cut holes H cannot be completely sealed by the sealing members. There is no other choice but to affix the sealing members by hands. Thus, a human error in the affixing may occur.
Furthermore, with a recent increase in an image forming speed, the rotating polygonal mirror needs to rotate at high speed. When the rotating polygonal mirror rotates at high speed, air current is generated in the optical scanning apparatus. The air current causes a large pressure distribution difference in an inner space of the optical scanning apparatus. Depending on areas, air flows from inside to outside or from outside to inside. A force caused by the air current is exerted on the optical scanning apparatus more greatly than that in a conventional example. Air flowing from outside to inside includes fine dust that soils the optical component. Thus, as the apparatus operates, more fine dust enters inward. The fine dust adheres to a surface of the optical component and the inside of the casing 185. Particularly, dust carried by surrounding air current and the rotating polygonal mirror rotating at high speed collide with each other. Thus, the dust is likely to adhere to a reflection surface of the rotating polygonal mirror. More specifically, when the rotating polygonal mirror rotates, a Karman vortex and turbulence are generated around the reflection surface of the rotating polygonal mirror, and the air current carrying the dust violently collides with the reflection surface. As a result, the fine dust is accumulated on the reflection surface of the rotating polygonal mirror. Thus, dirt may increase from an area where an amount of collision of the air current is large, so that the reflectance of the reflection surface decreases. The amount of the light beam to be guided onto the photosensitive member decreases, so that the image density decreases.
To solve such an issue that the optical component is soiled, a configuration in which the die cut holes H are covered using a part of the plate spring installed inside the casing 185 has been proposed. According to an optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2008-9319, for example, an opening portion to be formed when a casing is formed is covered by bending a tip end of a plate spring to ensure sealability of the apparatus.
In a configuration discussed in Japanese Patent Application Laid-Open No. 2008-9319, unlike a material such as a rubber material or a sponge material, hard materials such as the plate spring and the casing are fitted to each other. Thus, it is difficult to completely cover the opening portion. If flatness and straightness of a contact surface between the plate spring and the casing 185 are distorted even by several micrometers, surface contact or line contact cannot be made but partial point contact is made between the plate spring and the casing. Thus, a gap is formed between the plate spring and the casing. In other words, a gap formed in a portion where the plate spring and the casing are in point contact may be unsealable. Therefore, dust of several hundred micrometers can be prevented from entering. However, fine particles in the order of several micrometers as described above cannot be prevented from entering.